EM telemetry is a method of transmitting data from a MWD assembly that resides just above an oil well drill bit. An electromagnetic telemetry system is a used for transmitting data from downhole assembly, which is operationally attached to a drill string, to a telemetry receiver system. The data are typically responses of one or more sensors disposed within the downhole assembly. A downhole transmitter induces a signal current within the drill string. The signal current is modulated to represent the transmitted data.
    EM-MWD uses electromagnetic waves to transmit downhole measured data in real time to the surface during conventional and under-balanced horizontal and directional drilling operations. EM telemetry transmits information through the formation to the surface antenna, where it is received and sent to a data acquisition system to be decoded and processed. The system is battery powered and can operate during all phases of drilling operations, including tripping, drilling, and lost circulation. The tool is particularly attractive in wells drilled with unconventional drilling fluids such as foam, mist, or aerated mud.